1. Field of the Invention
The present invention relates to a control circuit for driving a print head of a printing apparatus, and more particularly, to a control circuit for driving a print head of a printing apparatus to make temperature compensation and provide uniform ink spots.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art print head 70. The print head 70 comprises an ink reservoir 72, a plurality of tubes 74 and a plurality of ink chambers 76. The plurality of tubes 74 connects the ink reservoir 72 to the plurality of ink chambers 76. Ink inside the ink reservoir 72 can flow through the tubes 74 to the ink chambers 76. Inside each ink chamber 76 is a heating resistor 78 that heats up the ink, increasing the ink""s thermal energy. When the thermal energy of the ink in the ink chamber 76 is above a predetermined threshold, the ink generates bubbles 80 to eject ink spots from a nozzle 82 for printing. When the nozzle 82 receives many instructions successively to eject ink spots, the heating resistor 78 of the nozzle 82 continually heats up, and ink inside the ink chamber 76 has a higher temperature and a lower viscosity. If, however, another nozzle 82 receives fewer instructions to eject ink spots, ink inside the ink chamber 76 has a lower temperature and a higher viscosity. If the same amount of energy is used to drive the heating resistors 78 of these two nozzles 82, non-uniform ink spots are ejected and the printing quality is lowered. So, it is preferable that the energy provided by the heating resistor 78 in the print head 70 not only makes the thermal energy of ink in the ink chamber 76 higher than the predetermined threshold, but can also be adjusted to make the sizes of ejected ink spots uniform so as to optimize printing quality.
Please refer to FIG. 2. FIG. 2 is a schematic diagram of a prior art driving circuit of a print head. For example, a driving circuit 10 can receive an input of eight printing data 30 and produce eight controlling signals (D1, D2, D3, D4, D5, D6, D7, D8) to output to a print head 40. The print head 40 has a heating circuit 42 and eight ink chambers (R1, R2, R3, R4, R5, R6, R7, R8). The driving circuit 10 has a shift register 22, a latching circuit 24 and a driving module 26. The shift register 22 receives binary printing data 30 transmitted serially from the printing apparatus. Then, the latching circuit 24 latches the printing data 30 and stores the printing data 30 in the latching circuit 24 according to a latching signal 34. The driving module 26 consists of a plurality of AND gates 28 and causes the heating circuit 42 in the print head 40 to heat up each predetermined ink chamber according to a driving signal 36. The heating circuit 42 consists of a plurality of heating resistors 78 and transistor switches 44. Each transistor switch 44 is linked from its corresponding control signal (D1, D2, D3, D4, D5, D6, D7, D8) to the AND gate it controls. When a specific control signal is turned on, the corresponding transistor switch 44 turns on, current flows through the corresponding heating resistor 78, the corresponding ink chamber is heated up, and ink inside the ink chamber is ejected as ink spots to print.
Please refer to FIG. 3. FIG. 3 is a timing diagram for a first driving pattern of a prior art print head. The thermal energy of ink inside the ink chamber 76 is influenced by energy provided by the heating resistor 78 and other factors, such as the number of ink chambers to be driven in a printing process. When there are more ink chambers to be driven in a printing process, the heating resistor 78 provides less energy to these ink chambers. Between T0 and T1, eight printing data 30 are input sequentially to the shift register 22 via the control of a pulse signal 32. When the latching signal 34 produces a pulse, binary bits of eight printing data 30 are respectively latched in the latching circuit 24. Between T1 and T2, a pulse 37 is produced in the driving signal 36. The AND gate 28 of the driving module 26 then decides whether or not to output the pulse of the corresponding driving signal 36, depending on whether the latched printing data 30 in latching circuit 24 is a xe2x80x9c1xe2x80x9d or a xe2x80x9c0.xe2x80x9d For example, between T0 and T1, the printing data 30 are (1, 1, 1, 1, 0, 0, 0, 0). When the pulse 37 of the driving signal 36 is produced between T1 and T2, the corresponding transistor switch is on and a current flows through the corresponding heating resistors to heat up the corresponding ink chambers (R1, R2, R3, R4) to eject ink spots. Other transistors that are off do not conduct, so the corresponding heating resistors have no current and the corresponding ink chambers (R5, R6, R7, R8) are not heated. As a result, no ink spots are ejected from those chambers.
Between T1 and T2, printing data is renewed to (1, 1, 1, 1, 1, 0, 0, 0). So, between T2 and T3, a pulse 38 of the driving signal 36 is produced and corresponding ink chambers (R1, R2, R3, R4, R5) are heated to eject ink spots. Other ink chambers (R6, R7, R8) are not heated, so they do not eject ink spots. The duration of pulses 37, 38, and 39 is the same, but their voltages are different. The voltage of pulse 38 is lower than that of pulse 37 because five ink chambers are driven with less energy provided by heating resistor 78 in the second printing process compared to four ink chambers driven with more energy in the first printing process. For the same reason, six ink chambers are driven with even less energy in the third printing process, so the voltage of pulse 39 is lower than the voltages of both pulses 37 and 38.
Please refer to FIG. 4. FIG. 4 is a timing diagram of a second driving pattern of a prior art print head. FIG. 3 showed a case where the printing data 30 is concentrated (1, 1, 1, 1, 0, 0, 0, 0). FIG. 4 is different in that the printing data 30 is more dispersed (0, 1, 1, 0, 0, 1, 1, 0), (1, 0, 0, 1, 0, 1, 0, 1). Because the prior art only considers the number of ink chambers to be driven to eject ink drops, the duration and voltages of pulses 47, 48, 49 of the driving signal 36, and the energy provided to heating resistor 78, are the same. In fact, the thermal energy of ink inside the ink chamber 76 is influenced by other factors, one being active ink chambers in proximity to reserved ink chambers. As shown in FIG. 4, the distribution of the active ink chambers in the first printing process is concentrated, so the thermal energy of ink inside these ink chambers is actually higher. However, the distribution of the active ink chambers in the third printing process is very dispersed, so the thermal energy of the ink inside these ink chambers is actually lower. This situation is not considered in the prior art as shown in FIG. 4. Ejected ink spots are still not uniform in size and the printing quality is influenced.
It is therefore a primary objective of the claimed invention to provide a control circuit for driving a print head of a printing apparatus to make temperature compensation and provide uniform ink spots.
According to the claimed invention, a control circuit for driving a print head of a printing apparatus is provided. The print head comprises a plurality of heating elements and a plurality of corresponding ink chambers. Each ink chamber is used for storing ink and has a nozzle. The control circuit includes a thermometer for measuring a temperature of the ink chambers, and a processor for generating a heating signal according to printing data transmitted from the printing apparatus to drive heating elements to heat ink chambers corresponding to nozzles which will jet ink drops, so as to cause the nozzles to jet ink drops. The processor also includes a pre-heating signal to drive the heating elements according to the temperature measured by the thermometer. If the processor is to generate the pre-heating signal, the processor will generate the pre-heating signal in addition to generating the heating signal so as to provide additional energy to drive the heating elements corresponding to the nozzles that will jet ink.
It is an advantage of the claimed invention that the control circuit makes temperature compensation according to a temperature of the print head and heat-accumulation weightings to make ejected ink spots uniform in size so as to improve printing quality of a printer.
These and other objects and the advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.